Anson Caralya
First Post
My original estimation is that he would be about CR 16 (as opposed to CR 20) with no magic items.
I would guess a complete lack of magic gear would be a -2 hit to CR. For a humanoid, I might say -3, but an undead already has a lot of special abilities and qualities that they don't need to get from items. So, if this is meant to be a very challenging encounter for 12th level characters, I'd go with a lich Wiz15 (15 + 2 -2 = 15, 3 over party's level).